NdFeB sintered magnets are used in a motor for hybrid cars and other machines, and the demand for such magnets is expected to continue to expand in the future. Accordingly, it is desired to further increase their coercivity HcJ. One commonly known method for increasing the coercivity HcJ of NdFeB sintered magnets is to replace some of the neodymium (Nd) atoms with dysprosium (Dy) or terbium (Tb) atoms. However, Dy and Tb are scarce resources and unevenly distributed in the world. Another problem is that the aforementioned replacement of the elements lowers the residual flux density Br and the maximum energy product (BH)max of the NdFeB sintered magnet.
Patent Document 1 discloses the technique of depositing at least one element selected from Nd, Pr, Dy, Ho and Tb on the surface of an NdFeB sintered magnet to prevent a decrease in the coercivity that occurs when the surface of the NdFeB sintered magnet is processed to make a thin film of it or for other purposes. Patent Document 2 discloses the technique of diffusing at least one element selected from Tb, Dy, Al and Ga in the surface of the NdFeB sintered magnet to suppress an irreversible demagnetization that occurs at high temperatures.
In recent years, it has been found that HcJ of an NdFeB sintered magnet can be increased, with only a minor decrease in Br, by adhering Dy or Tb to the surface of the magnet and then heating it at 700° to 1000° C. (Non-Patent Documents 1 to 3). Dy or Tb adhered to the magnet's surface is transported through the grain boundaries of the sintered body to the inner regions thereof, and diffuses from the grain boundaries into each grain of the main phase, i.e. R2Fe14B, where R is a rare-earth element (which process is called the “grain boundary diffusion”). In this process, since the R-rich phase of the grain boundaries is liquefied by heat, the diffusion rate of Dy or Tb through the grain boundaries is much higher than their diffusion rate from the boundaries into the grains of the main phase. By utilizing this difference in the diffusion rate and appropriately regulating the temperature and time of the heat treatment, one can create, over the entirety of the sintered body, a state where the concentration of Dy or Tb is high only in the region of the grains of the main phase the boundaries (surface region) in the sintered body. The coercivity HcJ of an NdFeB sintered magnet depends on the condition of the surface region of the grains of the main phase; the presence of crystal grains with high Dy/Tb concentrations in the surface region provides the NdFeB sintered magnet with a high coercivity. Although the increase in the concentration of Dy or Tb decreases Br of the magnet, the decrease in Br of the whole main-phase grain is negligible since it occurs merely in the surface region of each grain of the main phase. In this manner, a high-performance magnet whose Br is comparable to that of the NdFeB sintered magnet with no element replaced with Dy or Tb can be produced. This technique is called the grain boundary diffusion method.
Some industrial methods for producing an NdFeB sintered magnet by the grain boundary diffusion method have already been published. One of those methods includes heating an NdFeB sintered magnet after forming a layer of fine powder of a fluoride or oxide of Dy or Tb on its surface; another method includes embedding an NdFeB sintered magnet in the mixture of a powder of a fluoride of Dy or Tb and a powder of calcium hydride, and heating the magnet in that state (Non-Patent Documents 4 and 5; Patent Document 3).
More recently discovered methods for attaining a high coercivity include depositing an alloy powder composed of Dy or Tb and another kind of metal (Patent Document 4), or depositing a mixture of a powder of a fluoride of Dy or Tb and one or more powders selected from Al, Cu and Zn (Patent Document 5), on the surface of an NdFeB sintered magnet body, and then performing the heat treatment.
Patent Document 1: Japanese Unexamined Patent Application Publication No. S62-074048
Patent Document 2: Japanese Unexamined Patent Application Publication No. H01-117303
Patent Document 3: Pamphlet of International Publication No. W02006/043348
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2007-287875
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2007-287874
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2007-053351
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-303435
Non-Patent Document 1: K. T. Park et al., “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets”, Proceeding of the Sixteenth International Workshop on Rare-Earth Magnets and their Applications (2000), pp. 257-264.
Non-Patent Document 2: Naoyuki Ishigaki et al., “Neojimu Kei Bishou Shouketsu Jishaku No Hyoumen Kaishitsu To Tokusei Koujou (Surface Improvements on Magnetic Properties for Small-Sized Nd—Fe—B Sintered Magnets)”, NEOMAX GIHOU (NEOMAX Technical Report), published by Kabushiki Kaisha NEOMAX, vol. 15(2005), pp. 15-19
Non-Patent Document 3: Ken-ichi Machida et al., “Nd—Fe—B Kei Shouketsu Jishaku No Ryuukai Kaishitsu To Jiki Tokusei (Grain Boundary Modification and Magnetic Characteristics of NdFeB Sintered Magnet)”, Funtai Funmatsu Yakin Kyoukai Heisei 16 Nen Shunki Taikai Kouen Gaiyoushuu (Speech Summaries of 2004 Spring Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, 1-47A
Non-Patent Document 4: Kouichi Hirota et al., “Ryuukai Kakusan Hou Ni Yoru Nd—Fe—B Kei Shouketsu Jishaku No Kou Hojiryoku-ka (Enhancement of NeFeB Sintered Magnet by Grain Boundary Diffusion)”, Funtai Funmatsu Yakin Kyoukai Heisei 17 Nen Shunki Taikai Kouen Gaiyoushuu (Speech Summaries of 2005 Spring Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, p.143
Non-Patent Document 5: Ken-ichi Machida et al., “Ryuukai Kaishitsu Gata NdFeB Sintered Magnet No Jiki Tokusei (Magnetic Characteristics of NdFeB Sintered Magnet with Modified Grain Boundary)”, Funtai Funmatsu Yakin Kyoukai Heisei 17 Nen Shunki Taikai Kouen Gaiyoushuu (Speech Summaries of 2005 Spring Meeting of Japan Society of Powder and Powder Metallurgy), published by the Japan Society of Powder and Powder Metallurgy, p.144